1. Field of the Invention
The invention relates to a fuel cell system that supplies a drive motor with electric power from a fuel cell generating electric power in an electrochemical reaction, and a method of controlling the fuel cell system.
2. Description of the Related Art
In recent years, fuel cells have been drawing attention as electric power supplies excellent in operation efficiency and environment friendliness. A fuel cell controls the amount of supply of fuel gas and outputs an electric power corresponding to a request from a drive motor. In some cases, however, as a result of a response delay in the amount of supply of gas, the responsiveness of the output electric power decreases, and a secondary battery is mounted as compensation. This secondary battery accumulates regenerative energy generated in reducing the speed of the drive motor and electric power generated by the fuel cell, and discharges the accumulated energy for the purpose of compensating for a decrease in the responsiveness of the fuel cell, increasing the output of an entire fuel cell system, and the like.
It should be noted herein that the fuel cell may be used in an extremely low-temperature environment. For example, in the case where an electrolyte membrane made of a polymer ion-exchange membrane is used, the interior of the electrolyte membrane needs to be held humid to generate electric power. In general, water is produced in the fuel cell through an electrochemical reaction. Accordingly, under a low-temperature environment, moisture in the fuel cell freezes, and there arises an apprehension that subsequent electric power generation through the electrochemical reaction may be hindered. Thus, there is disclosed an art for reliably generating electric power in a fuel cell even under a low-temperature environment (e.g., see Japanese Patent Application Publication No. 2006-156181 (JP-A-2006-156181). In this art, when the fuel cell is started under a low-temperature environment, the electric power generation characteristic in starting the fuel cell is improved by making the pressure of supply of gas higher than during normal electric power generation to improve the diffusion of gas supplied to gas diffusion layers in the fuel cell.
When the fuel cell is started under a low-temperature environment, the electric power generation efficiency of the fuel cell is conventionally lowered purposely to raise the amount of heat generation in the fuel cell itself and thus warm up the fuel cell itself. However, conventionally, the operation state of the fuel cell during warm-up is not mentioned, and the amount of heat generation in warming up the fuel cell is set as large as possible to prevent the fuel cell from failing to be started below freezing point. Thus, an excessive amount of energy is put into the fuel cell, and there is a high possibility of wasteful energy consumption.
Further, even when the fuel cell is started, there may be a desire to swiftly drive the drive motor supplied with electric power from the fuel cell. However, with a view to preventing the fuel cell from refreezing during start-up below freezing point, priority is given to warm-up until the temperature of the fuel cell rises to or above 0° C., and the drive motor is supplied with electric power after the completion of the warm-up. That is, a certain length of time is required until the drive motor is driven.